The present invention relates to a thermocontrol method for an injection molding machine, more precisely relates to a method for controlling temperature of thermocontrolled components, e.g. an injection cylinder, of the injection molding machine having means for heating and/or cooling, which are arranged at the prescribed intervals, to correspond to the instant operating status thereof.
Conventionally, the PID (Proportional band-Integral time-Differential time) control method is usually adopted to control the temperature of thermocontrolled components, e.g. an injection cylinder, of an injection molding machine.
The PID control method is based on a proportional action (P), which is in proportion to a control deviation; an integral action (I), which is based on an integrated value of the control deviation; and a differential action (D), which is based on differential coefficients of the thermocontrolled components. Using the PID control method, the object temperature can be maintained when the thermocontrolled components are under certain stable conditions.
However, the injection molding machine has various operating states such as stop, temperature rise, mold, pause, etc. Furthermore, heating elements and cooling elements are different devices in each status. For example, in the temperature rise state, the heat of the heaters is the main heating element; radiant heat is the main cooling element. On the other hand, heat from the heater, the heat of friction caused by an injection screw acting upon the resin, etc. are the heating elements in the mold status, while the natural radiant heat, the endothermic reaction by resin supplied, etc. are the cooling elements therein.
The interior temperature of injection cylinders is controlled to maintain a prescribed temperature distribution so as to inject resins smoothly. Generally, a plurality of heaters are arranged in the axial direction of the injection cylinder at prescribed intervals. Heat of each thermocontrolled component, which is conducted via separating parts between adjacent thermocontrolled components, affects the temperature of adjacent thermocontrolled components.
In the conventional PID control of the injection cylinder, the heating elements and the cooling elements are different in each status. The temperature of each thermocontrolled component of the injection cylinder is shown in FIG. 7.
FIG. 7 shows temperature change of each thermocontrolled component of the injection cylinder in the status of temperature rise, wherein the injection cylinder has three heaters which are serially provided between an injection nozzle and a hopper.
In the injection cylinder, the relationship among the object temperature T01 of a first thermocontrolled component whose temperature is controlled by a first heater H1 provided on the injection nozzle side; object temperature T03 of a third thermocontrolled component whose temperature is controlled by a third heater H3 provided on the hopper side; and object temperature T02 of a second thermocontrolled component whose temperature is controlled by a second heater H2 provided between the first heater H1 and the third heater H3 is: EQU T01&gt;T02&gt;T03
The temperature of an injection cylinder, for example, may be higher (overshoot, P.sub.o) or may be lower (undershoot, P.sub.u) than the object temperature, as shown in FIG. 7: A Temperature Graph of the Injection Cylinder.
Moreover, the second thermocontrolled component, which is controlled by the second heater H2, is affected by heat conducted from the first heater H1 and the third heater H3, so that the overshoot P.sub.o and the undershoot P.sub.u in the graph T2 of the second thermocontrolled component is greater than that of the other graphs T1 and T3.
The overshoot P.sub.o and undershoot P.sub.u of the injection cylinder have a large effect on the viscosity of molten resin, so that they may become factors in inferior production quality.
Moreover, the overshoot P.sub.o causes resin deterioration when the object temperature is exceeded and the resin deterioration temperature is attained, resulting in inferior products. To avoid inferior production quality, manual control of the temperature of the injection cylinder, based on the experience of a skilled operator, is required.